1. Field of the Invention
This invention relates in general to a magnetic read sensors, and more particularly to a method and apparatus for providing a spin valve transistor with differential detection.
2. Description of Related Art
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device, such as a disk drive, incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads carrying read sensors are then used to read data from the tracks on the disk surfaces.
An MR sensor detects a magnetic field through a change in resistance in its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin dependent scattering, which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe—Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field).
In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. It should be noted that the AMR effect is also present in the SV sensor free layer and it tends to reduce the overall GMR effect.
The magnetic moment of the free layer when the sensor is in its quiescent state is preferably perpendicular to the magnetic moment of the pinned layer and parallel to the ABS. This allows for read signal asymmetry upon the occurrence of positive and negative magnetic field incursions of a rotating disk.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage.
As systems are pushed to higher read density, higher magnetic bit size or decreased recording media size, the available magnetic flux is decreased. In addition, sensitivity may be decreased from thermal noise. For example, while the head is flying over the disk surface, it may hit a particle (contamination). The energy of this collision will be dissipated in the form of heat causing the temperature of the head to increase, causing an increase in the resistance of the head ultimately resulting in a signal that may be even higher than the magnetic signal from a transition. In order to sense these smaller signals and increase areal density, read heads with greater sensitivities are needed.
A scheme to increase the signal to noise ratio of a spin valve head is to employ first and second spin valve sensors, which are differentially detected for common mode noise rejection. A differential spin valve structure employs first and second spin valve sensors that produce responses of opposite polarities in reaction to a magnetic field of a single polarity. The opposite polarity responses are processed by a differential amplifier for common mode rejection of noise and for producing an enhanced combined signal. The first and second spin valve sensors are magnetically separated by a gap layer. The first spin valve sensor is connected in series with first and second leads and the second spin valve sensor is connected in series with third and fourth leads. The second and fourth leads are electrically interconnected and the first and third leads are adapted for connection to the differential amplifier.
Differential GMR and MTJ sensors comprising dual SV or MTJ sensors, respectively, can provide increased magnetoresistive response to a signal field due to the additive response of the dual sensors connected in a differential circuit. However, even greater increases in magnetoresistive response may be obtainable from yet another type of GMR sensor known as a spin valve transistor (SVT) sensor.
In one type of a spin-valve transistor, electrons are injected from an emitter via a tunnel junction into a base. This spin-valve transistor has a stacked structure of an emitter, a tunnel insulator, a base, and a collector. On the other hand, in another type of spin-valve transistor, electrons are injected from an emitter via a Schottky junction into a base. The spin-valve transistor is designed to operate based on spin-dependent scattering of electrons, which means that the manner of electron scattering changes depending on whether the spin directions are parallel or antiparallel in the two magnetic films of the spin-valve film included in the base. These spin-valve transistors are known to exhibit an extremely high MR ratio.
When a (100)-oriented spin-valve film having a stacked structure of a magnetic layer/a nonmagnetic layer/a magnetic layer is employed as a base of the spin-valve transistor, it is possible to increase a ratio of collector current/emitter current (Ic/Ie) while retaining a high MR ratio. When the base including the (100)-oriented spin-valve film is used, the diffusive scattering can be suppressed, and instead, ballistic conduction or interface reflection of electrons is caused at the interface of magnetic layer/nonmagnetic layer, depending on whether the spins of the two magnetic layers are parallel or antiparallel. Namely, if a magnetic layer sufficiently thin as compared with an electron mean free path in the magnetic layer is used and a flat interface between magnetic/nonmagnetic layers is formed so as to generate the ballistic conduction or interface reflection of electrons, it becomes possible to provide a transistor that exhibits a high ratio of Ic/Ie as well as a high MR ratio.
Intensity of interface reflection of electrons at the magnetic/nonmagnetic interface varies depending on the band structures in the magnetic and nonmagnetic layers. Since electrons can travel between bands having the same symmetrical property without being reflected, the up-spin electrons having higher energy than the Fermi level and moving in the [100]-direction can pass through the magnetic/nonmagnetic interface. On the other hand, since electrons cannot travel between bands having a different symmetrical property, the down-spin electrons will be strongly reflected.
Thus, it is important that the magnetic layer and nonmagnetic layer forming the spin-valve film included in the base are (100)-oriented. However, it is difficult to grow a (100)-oriented metal film on a IV-group semiconductor such as Si and Ge. On the other hand, it is known that a (100)-oriented metal film can be grown easily on a III–V compound semiconductor, such as GaAs.
In a differential sensor, however, for longitudinal recording on a disk, the bit configuration is arranged to have a bit transition length equal to the separation between a first free layer and a second free layer. In the presence of the signal fields, the free layers rotate in opposite directions resulting in additive signals from the first and second spin valve structures due to the antiparallel orientations of their pinned layers. Similarly, in the case of perpendicular recording, the bit configuration may be arranged to have a bit transition length equal to the spacing between the first and second free layers resulting in opposite first and second magnetic signal field polarities under the first and second free layers, and therefore additive signals from the first and second spin valve structures.
Therefore, it can be seen that higher areal densities are possible by minimizing the distance between the free layers in the differential sensor. However, one problem with spin valve transistors used in sensor applications is the use of the thick semiconductor substrate, such as GaAs that acts as the collector. The thick semiconductor substrate does not allow the use of a thin gap between the shields and therefore limits the minimization of the distance between the free layers in the differential sensor.
It can be seen that there is a need for a method and apparatus for providing a spin valve transistor with differential detection that avoids the use of shields so that the distance between the free layers in the differential sensor is minimized.